Subsea motor-turbomachine

ABSTRACT

Subsea pressure booster comprising an electric motor ( 2 ) and a compressor ( 3 ), multiphase pump or pump driven by the electric motor, for boosting the pressure of a subsea flow comprising petroleum gas or liquid, distinctive in that the pressure booster comprises a common pressure housing ( 1 ) for the electric motor and compressor, multiphase pump or pump; a diaphragm separating the common pressure housing into two compartments, one compartment for the electric motor and one compartment for the compressor; multiphase pump or pump; a shaft connecting the electric motor to the compressor, multiphase pump or pump; and at least one oil lubricated mechanical shaft seal arranged between the shaft and diaphragm; and bearings on the shaft, preferably at least two radial and one axial oil lubricated bearings ( 4, 4′, 4 ″) for an oil filled or gas filled motor compartment or magnetic bearings or oil lubricated bearings for a gas filled motor compartment.

FIELD OF THE INVENTION

The present invention relates to subsea rotating equipment, i.e. compressors, multiphase pumps and liquid pumps driven by electric motors, and its purpose is particularly to improve the motor internals of a gas filled motor, optionally a liquid filled motor, from contamination of the compressed or pumped fluids that could harm or damage the motor. This is obtained by separating the motor and compressor, multiphase pump or pump into two compartments by oil lubricated mechanical seals.

DESCRIPTION OF THE INVENTION, ITS BACKGROUND AND PRIOR ART

The invention protects the lube oil for the mechanical seal and bearings by prevention against gas contamination or other contamination by mechanical seals and over pressure of the lube oil compared to the internal pressure of the motor and pump. Control of pressure of the lube oil, motor atmosphere and compressor and suction pressure of the compressor, multiphase pump or liquid pump relative to each other is a key feature of the invention

In the following description and patent claims will the word “compressor” also include multiphase pump and liquid pump when relevant for all of them. If there are elements of the description and claims that are relevant for only compressor, multiphase pumps or liquid pump, this will be specifically described.

The invention and several embodiments of the invention are defined by the claims, to which reference is made.

Currently only liquid (e.g. oil, or water or MEG or mixtures thereof. In the following the term “oil” will also include all other suitable liquids for bearings, mechanical seals and gear) filled motors have been used for subsea multiphase and liquid pumps. This gives a practical limitation of the power of the motor at say 4 MW and a speed of say 4000 rpm due to the frictional losses when the rotor is rotating in liquid. More power could be provided by making such motors longer, but a practical limit of length seems to be reached at around 4 MW due to rortodynamics, required number of bearings and need for space. There are identified application for subsea multiphase and liquid pumps that requires more than 4 MW, typically 6 MW, and there is therefore a need for subsea gas filled motors that can be used for such pumps because such motors can be designed for even much more than 6 MW, e.g. 25 MW which is much more than needed for pumps, and this will remove the motor power limitation for sizing of pump capacity.

With respect to subsea compressors, a prototype, the Kvaerner Booster Station (KBS) compressor with a 850 kW motor was designed in the early 1990 and thoroughly tested through 1993, and also refurbished tested in 2002. This compressor, which additionally to motor and compressor had a step-up gear, had oil lubricated bearings and gear box and the seal between the motor and compressor compartment was a labyrinth seal. The lube oil circuit was open, i.e. the lube oil had direct contact with the raw gas atmosphere with its contaminants (e.g. particles, water vapour, H₂S and CO₂) that made the lube oil susceptible to degradation and damage of function. Further it is a weakness of a labyrinth seal between the motor and the compressor that it does not completely segregate these two compartments, and over time there will be some ingress of raw gas into the motor atmosphere resulting in risk of degradation and damage of internal materials of the motor and of the non metallic materials in particular caused by contaminants in the raw gas. There is therefore a need for a technical seal solution that conceptually completely or nearly prevents that raw gas flows into the motor, and further there is a need for a lube oil system that is kept separated from the raw gas to eliminate the risk of degradation of the lube oil, and this is according to the invention provided by lubricated mechanical seals.

There are several patents for the KBS type of subsea compressor. Reference is made to patents NO162782, NO172075, NO172076, NO172555, NO172566 and NO173197. The technically solutions described in these patens differ significantly from the invention by applying labyrinth seal and open lube oil circuit resulting in the risks and problems that are described above.

After the year 2000 some compressor manufacturers, subsea system suppliers and oil companies have carried out significant work to develop, design, construct, test and qualify subsea compressors with magnetic bearings to avoid the potential problems that are described above, caused by having a lube oil system. Such solutions, still being in the development phase and not fully qualified for subsea use, have also inherent potential problems. The polymeric surface materials of the bearings are susceptible to chemical attack from the raw gas with its contaminants when the motor atmosphere is raw gas, which is a common solution. This can in principle be resolved by covering the surface with a thin metal sheet, i.e. so called can bearings. This is however not considered to be a a proven solution, and it adds complication and risk in making the bearings and operation and control of them. The control system for subsea magnetic bearings that has to be located subsea and close to the bearings, is also a concern because at the current stage of technology it is being developed and qualified. Even a successful qualification will leave some risk concerns with respect to subsea operation because the electronics of the control system has a certain failure rate and the control system has quite large dimension and are heavy and therefore not easy to replace if it fails. There are also several wires with connectors and penetrators that also have their failure rate. All the concerns of this control system will be eliminated by using oil lubricated bearings according to the present invention because the lubrication system does not have any control system. A lube oil system, lubricated bearings and mechanical seals also have a risk of failing. In this case all components that together form the complete system are well known for different industrial applications and have a known failure rate that can be minimised by selection of quality of components. The novelty of the invention, compared to the mentioned industrial applications, is however the special configuration and adaptation to suit for the subsea conditions.

Reference is made to patents NO323324, NO323240, NO329089 and NO325900.

FIGURES

The invention is illustrated with FIGS. 1-6, illustrating embodiments and details of embodiments of the invention.

SUMMARY OF AND EMBODIMENTS OF THE INVENTION

The invention provides a subsea pressure booster comprising an electric motor and a compressor, multiphase pump or pump driven by the electric motor, for boosting the pressure of a subsea flow comprising petroleum gas or liquid, distinctive in that the pressure booster comprises a common pressure housing for the electric motor and compressor, multiphase pump or pump; a diaphragm separating the common pressure housing into two compartments, one compartment for the electric motor and one compartment for the compressor; multiphase pump or pump; a shaft connecting the electric motor to the compressor, multiphase pump or pump; and at least one oil lubricated mechanical shaft seal arranged between the shaft and diaphragm; and bearings on the shaft, preferably at least two radial and one axial oil lubricated bearings for an oil filled or gas filled motor compartment or magnetic bearings or oil lubricated bearings for a gas filled motor compartment.

The invention provides a simple and reliable solution for a subsea pressure booster for boosting the pressure of oil and gas from subsea wells and systems, allowing a gas filled motor compartment also for a pump and a multiphase pump, allowing higher effects than previously achievable without decreasing the reliability, with simple solutions for control of pressure, fluid integrity and operation. Since also oil filled motor compartments can be used, the definition of the invention as given above do not specify gas filled motor compartment as obligatory, which is because also subsea pressure boosters with oil filled motor compartment will benefit from the solutions provided with the present invention with respect to reliability and simplicity, particularly at high effects. However, inert gas, that is nitrogen, noble gas or in any case gas less aggressive than the raw gas according to prior art, is prefereably used in the motor compartment, using the solutions as described and illustrated in this document.

In the following will be described several embodiments of the invention that all have in common that they completely, or for all practical significance close enough to completely, protect the motor and lube oil for seals, bearing and gear (optionally) from being degraded and damaged by being contaminated by raw gas or pumped multiphase or liquid, that intermingles with the gas of the motor atmosphere or lube oil. This is obtained by control of the pressure level of lube oil, motor atmosphere and suction pressure relative to each other and by some way of ensuring that the motor atmosphere is inert and dry.

Different embodiments will be described by reference to figures. Explanation of the reference numbers of the figures are described in Table 1 below.

TABLE 1 Explanation of reference numbers of the FIGURES Reference no. Explanation  1 Common pressure housing for compressor and motor  2 Electric motor  3 Compressor or multiphase pump or liquid pump  4 Oil lubricated bearing, either radial or axial or combined  5 Housing for bearing  6 Oil lubricated mechanical seal  7 Diaphragm, partition  8 Impeller  9 Balance drum leakage, labyrinth 10 Balance drum 11 Coupling, rigid, flexible or common shaft for motor and compressor 12 Compressor inlet 13 Compressor outlet 14 Pump for lube oil circuit 15 Fan for motor gas recirculation through gas cooling circuit 16, 16′ Pressure transmitting tube from compressor to motor 17, 17′ Pressure transmitting tube from compressor to lube oil tank 18 Cooler for pressure transmitting gas 19 Valve for MEG supply (or other hydrate preventing chemical: e.g. DEG, TEG, methanol) 20 Lube oil pipe 21 Lube oil cooler 22 Valve for lube oil supply 23 Lube oil reservoir 24 Drainage line from motor 25 Drainage tank 26 Drainage valve; pressure relieve valve for motor gas 27 Pipe for leaked oil to discharge point at lower pressure than pm, e.g. compressor inlet or upstream separator 28 Gas cooling circuit pipe 29 Gas cooler 30 Anti-surge line for compressor or recirculation line for pump 31 Anti-surge cooler or recirculation cooler 32 Lube oil tank 33 Drainage direction of condensed liquid 34 Balance pipe 35 Level sensor 36 Level indication in drainage tank, high 37 Level indication level indicator in drainage tank, low 38 Level indication in lube oil tank, high 39 Level indication in lube oil tank, low 40 Nitrogen tank 41 Control valve for motor pressure pm 42 Control valve for lube oil pressure p″ 43 Valve for refilling of compressed nitrogen by umbilical or ROV or tube from ship connected by ROV 44 Pressure transmitter of pm 45 Pressure transmitter of p″ 46 Lube oil filter (optional) 47 Step-up gear 48 Anti-surge/re-circulation valve 49 Rotor 50 Stator 51, 51′ Membrane, e.g. floating piston 52, 52′ Shafts h Static height from bearings to low level of lube oil tank h′ Static height at low level of lube oil tank, variable from low to high level pd Discharge pressure pl Lube oil pressure in bearings pl′ Lube oil pressure at low level of lube oil tank pm Pressure of motor atmosphere pn Pressure of compressed nitrogen (or other actual inert gas) in tank. ps Suction pressure pt Pressure in drainage tank; approximately pt = pm p1 Pressure of first impeller or any other impeller p2 Pressure of second impeller or any impeller or discharge pressure, and impeller # is higher than the impeller that gives p1

The basic principle of the invention is to prevent communication of flow between compressor and motor. This is obtained by separation of the motor and compressor in two compartments, a motor and a compressor compartment, by a diaphragm with oil lubricated mechanical shaft seals, and by controlling the pressure of the lube oil such that the pressure of this in the seal is higher than both the pressure of the gas atmosphere of the motor and of the gas pressure of the suction side of the compressor.

It is important to note that the gas pressure in the balance drum of the compressor is suction pressure, which is obtained by a gas leakage from the discharge side of the compressor through a labyrinth seal and a balance pipe to the suction side of the compressor.

Mechanical seals of the invention will close by some kind of spring mechanism when the compressor is not operating, and therefore prevent intermingling of raw gas into the motor atmosphere at standstill, and it will also prevent loss of lube oil by leakage. By placing the lube oil tank at some elevation above the motor compressor, there will be a somewhat higher pressure of the lube oil inside the seals compared to the pressure inside the motor and compressor and hence any leakage will be from the lube oil side of the seals to the motor and compressor side, and thereby prevent lube oil from being contaminated.

The same principles with balance drum can be used for multiphase and liquid pumps. If other principles are used, e.g. opposed impellers for a pump or by having the pressure side of a multiphase pump or a pump at the motor, the pressure of the lube oil must be set such that it is higher than the highest fluid pressure at any of the seals.

The gas that forms the motor atmosphere, if not the gas being compressed, preferably is selected such that it is inert with respect to the motor materials including both metallic and non metallic materials. The gas can for example be dry nitrogen or dry hydrocarbon gas.

In the following will be described the invention with reference to FIGS. 1 to 6 and Table 1.

In the solution described in FIG. 1 the motor 2 is filled by raw gas from the compressor, preferably dry, by a pressure transmitting tube 16 from one of the compressor stages or from its suction side.

There is communication between the gas inside the motor and the gas flowing through the compressor, but because there is no flow through the motor, the motor is in principle filled with the same gas volume without exchange of atmosphere meaning that the amount of contaminants is limited to one volume filling and there is not a continuous supply of contaminants like it would be if it was a flow of raw gas through the motor or if there was a labyrinth seal between compressor and motor, which is quite open for intermingling.

In the illustration of FIG. 1 motor pressure pm is supplied from the stage 8 ¹ with pressure p1. Hence the pressure pm in the motor room will be equal to the first compressor stage pressure p1 and higher than the suction pressure ps. Drying of the gas is obtained by cooling the gas to seawater temperature or close to sea water temperature by heat exchanging to seawater by a heat exchanger 18 common for tubes 16 and 17′ in tube 17. The arrangement shown in FIG. 1 with a common cooler for tube 17 and 17 is of practical reasons. It could also be two separate tubes from stage 8 ¹ each with their cooler. In cases where cooling is found unnecessary, the cooler is omitted. Some amount of MEG (or other suitable hydrate preventing chemical) can if necessary be injected into the cooler 18 to prevent hydrate formation. The injected MEG will also have the favourable affect of drying the gas with respect to water, and also contribute to some MEG vapour pressure in the gas atmosphere. The combined result of these two mechanisms will be lowering of the water dew point of the gas, say by 5° C. During steady state operation there is no flow in the tube 16, and the gas will be cooled to seawater temperature. During transient operations, e.g. ramping speed of the compressor up and down or starts and stops, temperature and pressure of compressor and motor will change and there will be some flow in the tube 16 and thereby some exchange of gas between motor and compressor. Worst case is when starting or stopping the compressor and the motor and compressor is warmed up from sea water temperature to operation temperatures or in the opposite direction from operation temperatures down to sea water temperature. The direction of the equalising flow during transients will be a result of the relative temperatures and volumes of the motor and compressor and other factors. These transient flows will however be small, and it is easy to dimension and design heat exchangers to obtain a gas temperature equal to or at the level of the sea temperature, say not more than 5° C. above sea water temperature, and hence the gas will be dry during operation when the motor is warm and probably also at standstill when cooled down to sea water temperature because the small amount of water vapour that might flow from the compressor through the tube 16 to the motor will be diluted by the dry gas in the motor. If condensation should occur it will be insignificantly small with respect to cause harm to the motor materials if the raw gas has insignificant amounts of acid forming components like H₂S and CO₂.

If the pressure pl″ is supplied from one of the compressor impellers and at a higher pressure than the motor pressure, the lube oil tank 32 can be placed besides or under the pressure housing 1 when the pressure p″ is high enough to lift the oil into the bearings and seals and give them an overpressure relative to the motor pressure and the compressor suction pressure.

A membrane 51 can be inserted in the pressure transmitting tubes 16, 17, 17′ to further prevent flow communication between compressor and motor rooms at transient conditions and also by diffusion at steady-state. The membrane can e.g. be of the floating piston type, bellow type or a flexible polymeric material. The volume capacity of the membrane 51 for gas expansion designed to accommodate the worst case of volume difference change between compressor and motor rooms and between compressor and lube oil tank.

In FIG. 1 the pressure above the liquid level in the lube oil tank will be the same as the motor pressure pm and equal to the first stage compressor pressure p1. The gas will be kept dry by the same heat exchanger as for the motor gas. If the lube oil tank 32 is elevated above the bearings 5, 5′, 5″ of the motor-compressor 1, 2, 3, the lube oil pressure in the bearings at standstill will be pl=pm+h+h′, and pm=ps, i.e. pl>pm=ps. During operation the pressure difference between pl and pm can be arranged to be higher than by standstill by supplying the oil from the pressure side of the lube oil pump 15. If the static height is large enough compared to pressure losses in the suction pipes and bearings, the bearing can be on the suction side of the pump. In the figure, the pump is connected to and run by the compressor shaft either directly or by some mechanical gear or transmission or by magnetic coupling or magnetic gear, but the pump can also be a separate pump arranged some place along the lube oil circuit 20 including inside the lube oil tank 32. When the pump is located outside the compressor in the lube oil circuit, it can be arranged to be separately retrievable and there can be several pumps to have redundancy. In case of redundant pumps arranged in parallel or series, there must be a valve arrangement, including check valves, such that it is possible to automatically continue operation with only one pump in operation, or the setting of valves can be arranged manually from surface (platform or onshore) or even by ROV. Consumption of oil by leakage through a seal is small, in the range of 1 litre per day, and for the five seals indicated in FIG. 1 there will be a total leakage per day in the range of 5 litres, i.e. around 2 m³ per year. By for instance dimensioning volume of the lube oil tank to 5 m³, it will only be necessary to top up the every second year by ROV or supply of oil by umbilical or even by exchange of the empty tank by a full tank. Another method could be to exchange the almost empty tank with a full tank at required intervals. Practical considerations including reliability and cost will be basis for selection of solution from case to case.

If lube oil is supplied from surface directly to the lube oil circuit without a buffer tank, there will be necessary to control the lube oil pressure pl by some remedy, e.g. a Pressure Volume Regulator (PVR) that is well known form applications for multiphase and liquid pumps. More traditional control of pl relative to pm and ps can also be applied, because there will normally not be need for very quick control to keep pl>pm and ps.

Practical considerations including reliability and cost should be the basis for selection of solution from case to case.

Drainage of the oil leaked into the motor from the seals 6 ¹, 6 ² and 6 ³, typically around 3 litres per day, is for the embodiment of FIGS. 1 and 3 easy during operation because the pressure pm=p1 of the drain tank 25 is higher than suction pressure ps, and the collected leakage oil can be routed to e.g. a separator, the balance gas line 34 or to the gas pipe upstream the compressor.

If for instance the volume of the drainage tank is 1001, there will in theory only to be necessary to drain in the range of once per month. The drainage can be arranged to be automatically by having some kind of level sensor 35 (pressure difference, nucleonic, other) that gives a signal to open the drainage valve 26 when a set high level 36 is reached as will as closing when a low level 37 is reached. Drainage at pre set intervals is also an option, however more risky and so is manual operation of valve 26 based on readings from level indicator. Control of the opening of the valve 26 is such that no gas blows out from the motor during drainage.

The valve 26 can also act to reduce the pressure of the motor if it during some mode of operation should be higher than desired. A signal from a pressure transmitter 44 that senses the motor pressure is then the basis for operation of valve 26.

In FIG. 2 is given an embodiment of the invention quite similar to the embodiment of FIG. 1, but the difference is that the pressure in the motor is pm=ps, and the gas pressure above the liquid level in the lube oil tank pl″=p1. In this case can be arranged heat exchangers (not shown) in the pressure transmitting tubes 16,17 with MEG injection if necessary similar to 18 in FIG. 1.

In the embodiment of FIG. 2, drainage to a point with pressure of ps will require some static over height which can be obtained by sufficient elevation of the drainage tank, which is below the motor 2, compared to the selected drainage point.

In FIG. 3 is illustrated an embodiment where pl (pressure in seals and bearings)=p2+h+h′+lube oil pump pressure>pm=p1.

FIG. 4 illustrates an embodiment that completely assures an inert motor atmosphere of nitrogen with small risk of contamination from the compressor atmosphere, which is a suitable solution if the raw gas contains significant contamination of aggressive components (e.g. H₂S and CO₂), and that also better assures a dry motor atmosphere and no condensation (hydrocarbons and water) if there is doubt about effect of the condensers 18, 18′. The atmosphere above the lube oil is protected by nitrogen in a similar way.

Compressed nitrogen (in the term nitrogen is also included any other suitable inert gas) at high pressure, i.e. at higher pressure than required to keep pl higher than both ps and pm, e.g. 300 bar or higher in the tank 40. Because the nitrogen consumption in theory is zero both at operation and standstill, the tank can be quite small, e.g. 2 m³ and still allow for long intervals between refilling. Refilling can be done either by exchanging the nitrogen tank, connect a pipe from a ship by ROV and supply from the ship or by tube in umbilical. Pressure of the motor pm is adjusted by supplying an amount of nitrogen that results in the desired pressure. To obtain this the motor must have a pressure transmitter, and the supply of nitrogen by valve 41 is controlled by the pressure, either by an automatic control or manually from surface. Control of the pressure pl″ in the atmosphere above the lube oil level in the lube oil tank is achieved in a similar way.

A mixed solution where the motor is supplied with nitrogen and the lube oil tank atmosphere with gas pressure from the compressor could also be applied in some cases, i.e. a combination of the embodiments in FIGS. 1 and 3 and FIG. 4.

Yet a solution, shown in FIG. 5, is to prime the motor by initially filling it with nitrogen, and then close the nitrogen supply valve and have pressure supply to the motor by some of the solutions of FIGS. 1 to 3. Some contamination of the motor atmosphere by intermingling of raw compressor gas caused by diffusion or by relative volume changes of motor and compressor room during transients, especially starts and stops, will not cause harm because condensation of water and hydrocarbons will be prevented by dilution of the contaminants and so will be possible content of H₂S, CO₂ and other harmful contaminants in the raw gas. At intervals or initiated by measurement of contaminants in the motor atmosphere, purging can be performed of the motor room by nitrogen to almost re-establish a pure nitrogen atmosphere. By inserting a membrane 51 in the pressure transmitting tube 16, contamination of the motor atmosphere by raw gas will be almost completely prevented.

Compressed nitrogen (in the term nitrogen is also included any other suitable inert gas) at high pressure, i.e. at higher pressure than required to keep pl higher than both ps and pm, e.g. 300 bar or higher can be supplied to the tank 40. Because the nitrogen consumption in theory is zero both at operation and standstill, the tank can be quite small, e.g. 3 m³ and still allow for long intervals between refilling. Refilling can be done either by exchanging the nitrogen tank, connect a pipe from a ship by ROV and supply from the ship or by tube in umbilical. Pressure of the motor pm is adjusted by supplying an amount of nitrogen that results in the desired pressure. To achieve this, the motor must have a pressure transmitter, and the supply of nitrogen by valve 41 is controlled by the pressure, either by an automatic control or manually from surface. Control of the pressure pl″ in the atmosphere above the lube oil level in the lube oil tank is obtain in a similar way.

Finally an embodiment is shown FIG. 6 where a mechanical step-up gear (either parallel or planet gear or any suitable type) with for instance a step-up of 3:1 is introduced between the motor and the compressor. This will allow that the Variable Speed Drive (VSD) for the motor can be surface located and still transmit electric power to the compressor motor over a long distance without Ferranti effect and instability by supplying the electric power at low frequency, e.g. at the level of 50 Hz±20 Hz, which gives a speed of a 2-polemotor of 3000±1200 rpm, and the compressor a speed can then be varied from 5400 to 12600 rpm in the case of a step-up of 3:1. Both transmission frequency and step-up ratio can be selected different from the example according to what is best suitable from case to case. Pressure control of lube oil and motor atmosphere can be according to any of the embodiments given presented in FIGS. 1 to 4.

A conceivable embodiment of the lube oil circuit could be an “open circuit” which implies that oil that leaks into the motor compartment and collected in the drainage tank 25 and pumped back into the lube oil circuit by some means. This can be obtained in different ways. A pump can for instance be installed to pump from the discharge of the tank 25 and into the lube oil circuit 20 or to the lube oil tank 32 or any other suitable point in the lube oil circuit. Alternatively the pressure pm in the motor can temporarily for a short period, say seconds, be set high enough to lift the oil in tank 25 into the lube oil circuit. The negative feature of this is possible contamination of the lube oil.

In all embodiments of FIGS. 1-6 a particle filter can be installed at some suitable point in the lube oil circuit. A bypass can be arranged with automatic change over to a shunt pipe around the filter in cases where the filter is clogged and the pressure loss over the filter becomes unacceptable high. The change over to the shunt pipe is controlled by the pressure differential over the filter.

The subsea pressure booster, turbomachine, compressor, multiphase pump or pump of the invention can include any feature as described or illustrated in this document, in any functional combination, each such combination is an embodiment of the present invention. The invention also provides use of the subsea pressure booster of the invention, for boosting the pressure of flow from subsea oil and/or gas wells or systems. 

1. A subsea pressure booster comprising: an electric motor; a compressor, multiphase pump or pump driven by the electric motor, for boosting a pressure of a subsea flow comprising petroleum gas or liquid; a common pressure housing for the electric motor and the compressor; a diaphragm separating the common pressure housing into a first compartment for the electric motor and a second compartment for the compressor; a shaft connecting the electric motor to the compressor; and at least one oil lubricated mechanical shaft seal arranged between the shaft and the diaphragm; and a plurality of bearings disposed on the shaft.
 2. The subsea pressure booster according to claim 1 wherein a lube oil pressure in the at least one seal between the first compartment and the second compartment is higher than both a compressor suction pressure and an internal motor pressure.
 3. The subsea pressure booster according to claim 2 wherein the internal motor pressure is a pressure of a first compressor impeller which internal motor pressure is transmitted into the electric motor by a tube.
 4. The subsea pressure booster according to claim 2 wherein the internal motor pressure is equal to the compressor suction pressure which internal motor pressure is transmitted to the electric motor by a tube.
 5. The subsea pressure booster according to claim 3 wherein a cooler is disposed in a tube or and arranged such that condensate does not flow into the electric motor but into the compressor.
 6. The subsea pressure booster according to claim 1 wherein gas pressure above a lube oil in a lube oil tank is equal to a pressure of a first impeller, which pressure is transmitted to the to the lube oil tank from the first impeller by a tube.
 7. The subsea pressure booster according to claim 1 wherein the pressure above a lube oil in a lube oil tank in the motor is supplied from a nitrogen reservoir.
 8. The subsea pressure booster according to claim 1 comprising an arrangement for handling oil drained out of the electric motor and the compressor.
 9. The subsea pressure booster according to claim 8 wherein oil that has leaked into the electric motor is pumped back to a lube oil circuit.
 10. The subsea pressure booster according to claim 1 wherein a oil lubricated step-up gear is arranged between the electric motor and the compressor on a motor side of the at least one seal.
 11. The subsea pressure booster according to claim 1, wherein the electric motor is operatively connected to drive the compressor directly via at least one of a common shaft, a coupling, or a gear coupling connecting a motor shaft to a compressor shaft; wherein the compressor comprises at least one oil lubricated mechanical shaft seal; wherein a lube oil pressure means providing control of a lubrication oil pressure the lubrication oil pressure is higher than or equal to a motor housing pressure and a compressor suction pressure at all operating conditions.
 12. The subsea pressure booster according to claim 1, wherein the motor housing is filled with a gas.
 13. The subsea pressure booster according to claim 12, comprising a motor gas means controlling a pressure and reducing a humidity of the gas.
 14. The subsea pressure booster according to claim 11, wherein the motor and the compressor are connected via at least one shaft; a common pressure housing encapsulates the motor and the compressor; a partition in the common pressure housing separates the common pressure housing into a motor housing and a compressor housing; a lubrication oil tank is arranged at a higher elevation than the common pressure housing; lubrication oil lines are arranged from a lower oil filled part of the lubrication oil tank to mechanical seals at either end of the shaft and at either side of the partition; a line is arranged from a first stage of the compressor to an upper gas filled part of the lubrication oil tank; the elevation of the lubrication oil tank and variable overpressure provided by the line from the compressor first stage provide an automatically adjustable lubrication oil overpressure; the motor housing is operatively connected to a lubrication oil drain tank; a pressure balancing line is arranged from the first stage of the compressor to the motor housing; and a gas drying device such as a sea water cooled heat exchanger and a MEG injection means is arranged in or operatively connected to the pressure balancing line.
 15. The subsea pressure booster according to claim 1, wherein the mechanical shaft seals comprise a spring or another biasing element which together with lube oil pressure means provide closing of the seal dependent on the rotations per minute (rpm) of the shaft, from a closed position at 0 rpm to a maximum open position at maximum rpm.
 16. The subsea pressure booster according to claim 1, wherein a lubrication oil tank has ROV-operable connections and lifting means in order to be replaceable by an ROV (Remotely Operable Vehicle).
 17. The subsea pressure booster according to claim 1, comprising lube oil pressure means as a separate lubrication oil tank arranged at a higher elevation than the compressor pressure housing for each mechanical seal or group of mechanical seals operating at common pressure; a lubrication oil line is via an oil filled part of a piston cylinder arranged from a lower oil filled part of the lubrication oil tank to the or each mechanical seal; a one way valve is arranged in the lubrication oil line between a cylinder and the lubrication oil tank; the valve is directed to open up for lubrication oil supply from the lubrication oil tank only when a seal pressure is below a tank pressure; a line is arranged from a first pressure stage of the compressor to an upper gas filled part of the lubrication oil tank; a line is arranged from a second pressure stage to a gas filled side of a piston cylinder, the piston cylinder may include a spring providing a minimum overpressure of the mechanical seal or the pressure of the second pressure stage minus the first pressure stage may provide the overpressure, said overpressure being in the range 40-0 bar.
 18. The subsea pressure booster according to claim 1, wherein a lube oil tank can be arranged beside or under the electric motor if pressure above a lube oil is supplied from a compressor impeller. 